How does SQL Server calculate the initial size of a (compressed) backup?

For compressed backup SQL server take the 1/3 size of the database. That can be vary once than final size.

for more details following links might be helpfull:

https://blogs.msdn.microsoft.com/psssql/2011/08/11/how-compressed-is-your-backup/

https://blogs.msdn.microsoft.com/psssql/2011/08/11/how-compressed-is-your-backup/